In a connection-oriented network with a Generalised Multi-Protocol Label Switching (GMPLS) control plane it is possible to establish a connection, called a Label Switched Path (LSP), between network nodes. It is desirable that a network is resilient to the failure of a span (link) between nodes, or to a node. GMPLS includes signalling extensions which support recovery. Recovery provides a way of detecting a failure on a working path, signalling the occurrence of the failure, and then transferring traffic from the working path LSP to a recovery path.
It is possible to recover an end-to-end Label Switched Path (LSP). This is called end-to-end recovery and is defined in IETF document [RFC4872] “RSVP-TE Extensions in Support of End-to-End Generalized Multi-Protocol Label Switching (GMPLS) Recovery”. It is also possible to recover a part of an end-to-end LSP. This is called segment recovery and is defined in IETF document [RFC4873] “GMPLS Segment Recovery”.
FIG. 1 shows a simple network 5 with nodes N1-N7. An end-to-end Label Switched Path (LSP) 10 connects node N1 to node N7 along the path N1-N2-N3-N4-N5-N7. N1 is called an Ingress Node and N7 is called an Egress Node. The network 5 has a recovery mechanism for a segment 11 of the LSP 10, between nodes N2 and N5, i.e. for segment N2-N3-N4-N5. The segment 11 has a segment recovery path 12 routed via node N6, i.e. path N2-N6-N5. Node N2 initiates the segment recovery path and is called a branch node. Node N5 terminates the segment recovery path and is called a merge node. Procedure and messages used to create and activate both the worker and the segment recovery LSP are described in [RFC4873] and [RFC4872]. Segment recovery provides resilience against a failure of spans N2-N3, N3-N4, N4-N5 or against failure of nodes N3, N4.
There are two mechanisms by which a failure in the network can be signalled. FIG. 2 shows the network 5 of FIG. 1 with a failure on the span between nodes N3 and N4. Firstly, a failure can be detected via a data plane Operation Administration and Management (OAM) mechanism and signalled, for example, by an Alarm Indication Signal (AIS) 13 which travels with the data along the working path 10. This is a quick mechanism and helps to ensure that a recovery operation can be concluded within a short time period, such as 50 ms. Secondly, a failure can be detected and signalled by a control plane mechanism. In FIG. 2, nodes N3 and N4 each detect a failure and send a signalling message 14, such as a Resource Reservation Protocol Traffic Engineering (RSVP-TE) Notify message, to the branch node N2. The control plane mechanism is useful in a network that does not have the capability of detecting failures directly from the data plane. The two mechanisms described here can co-exist. This means that branch node N2 may receive data plane signalling 13 and control plane signalling 14 in response to a failure of a span or node. In FIG. 2, the branch node N2 is in charge of activating the segment recovery procedure.
FIG. 3 shows the network of 5 of FIG. 1 with a failure on the span between nodes N5 and N7. Here, still considering the case of a bi-directional working path 10, the branch node N2 detects the failure of the working LSP 10 via data plane signalling (e.g. AIS) 13 and therefore will start the activation of segment recovery LSP 12 (N2-N6-N5). However, it can be seen that this recovery operation has no useful effect. Following transfer of traffic to the segment recovery path 12, there is still an end-to-end LSP between N1 and N7 which includes the failed section N5-N7. A further disadvantage of this behaviour is that the data plane failure detection signalling can be forwarded by node N5, along the recovery LSP 12, to node N2, causing node N2 again to perform a recovery operation back to the working path 10. This undesirable cycle of behaviour could be repeated a high number of times, preventing traffic from reaching the egress node N7.